1. Field of the Invention
This invention relates to a method of continuous fabrication of mother-capacitors.
2. Description of the Prior Art
A mother-capacitor is the term used to designate a capacitor in the form of a parallelepipedal bar, the side faces of which are spray-coated by the Schoop process. The bar is obtained from a metallized plastic film arranged in a flat stack or wound on a wheel of large diameter. A large number of individual capacitors are fabricated from this capacitor by cutting this latter at right angles to the spray-coated side faces. Different methods for the fabrication of mother-capacitors of the type mentioned above are described for example in French patents Nos. 879,280, 903,040, 1,051,464, 1,051,465 and 2,011,553.
In order to achieve a sufficiently high standard of quality and production efficiency, this technology entails the need to form the winding from metallized film of identical width with a very high degree of accuracy. Furthermore, the development of miniaturized capacitors makes it necessary to ensure that these widths of metallized film are of low value, namely of the order of 5 mm. In consequence, the non-metallized borders of these films must also be of small width in order to avoid any reduction in capacitance of the capacitors thus formed.
Finally, the heights attained by metallized film having widths of this order, whether stacked or wound on a wheel of large diameter, do not permit high production efficiency by reason of the fact that any increase in height is attended by a not-negligible danger of collapse of the mother-capacitor.
These different requirements lead to the conclusion that they cannot all be satisfied at the same time by the known methods at present in use. In point of fact, the metallization of films and formation of non-metallized borders on these films (by means of techniques which are well-known to those versed in the art such as a movable mask or oil mask) are carried out in a metallization unit whereas the operations involved in cutting and rewinding of strips in the form of thick disks are carried out in a cutting unit. Experience has shown that films of identical width with a narrow border can be obtained simultaneously only with very great difficulty. In order to achieve such a result, it would be necessary in the first place to obtain a perfect match between the positioning of the border-forming tools in the metallization unit and positioning of the cutting tools in the cutting unit. In the second place, it would be necessary to ensure perfect dimensional stability of the dielectric during these different operations performed under mechanical and/or thermal stress. It is found in particular that, in the case of fixed cutting knives, it is necessary to exert a tractive effort on the bordered films in order that the knives may cut in said borders with varying degrees of success (as the total width of the film increases, so the problems become more numerous).
Since these two requirements are not satisfied in practice, utilizable films can be obtained only at the cost of certain sacrifices from a productivity standpoint, viz:
the total width of film at the level of the metallization is reduced in order to avoid accumulation of mechanical errors in the tolerances of positioning of the border-forming and cutting tools at the level of the stack;
the tolerance allowed on the width of films is established first in order of priority and the minimum width of the non-metallized borders is deduced therefrom. In other words, the width of the borders may exceed the minimum value, thus inducing a loss of capacitance at the level of the capacitor thus formed (or a greater volume in respect of a given capacitance.
In all cases, the mother-capacitors formed by stacking or winding on a wheel of large diameter are fabricated from films which have previously been wound in the form of thick disks. In the case of narrow film widths (of the order of 5 mm), the maximum possible diameter of said thick disks is very lixited since it is soon found that there is a considerable risk of "telescoping" or collapse of the disks. In consequence, production rates decrease as a function of the frequency with which said thick disks are changed.